With the growing energy shortages on earth, the development of renewable (green) energy systems such as solar power generation systems is in an urgent need, wherein more efficient use of electrical energy is a key in promoting the progression of solar power generation system.
Isolated DC-DC converters are widely used in solar power generation systems particularly for medium or low power conversion, due to their good performance in voltage regulation and electrical isolation, as well as providing high reliability.
In traditional power converters, power switches are turned on/off through hard switching. Due to the transformer leakage inductance and stray capacitance of the circuit wirings, hard switching tends to have issues such as high conversion loss, noise, switching stress and other issues. With regard to a metal-oxide-semiconductor field-effect transistor (MOSFET), if hard switching is used, when the switching element is turned off, the voltage at either end of drain-source will rise quickly and produce a voltage surge, such that the MOSFET will encounter a voltage exceeding the normal input range. The voltage stress of the switch element is thus increased. When the switching element is turned on, the switching element will generate a current surge that is larger than the normal current at the instant of conduction owing to the influence of the bypass capacitor of the MOSFET itself, thereby increasing the current stress of the power element.
In order to improve the issues brought about by hard switching, clamp circuits are developed to reduce the voltage and current stresses of the power switches. However, some issues still exist in the following three common ways of implementing a traditional clamp circuit: (1) the recovery of energy of a RCD clamp circuit is absorbed by its resistors, thereby reducing the overall efficiency of the circuit; (2) it is difficult to implement an LCD clamp circuit, although it can achieve zero-voltage soft switching and energy recovery since timing of resonance of the inductor and the capacitor has to be taken into account, and furthermore, in circumstances of high frequency and large current, the resonant current will result in very high conduction loss in the circuit; and (3) an active clamp circuit is consisted of an active switching element and a capacitor, its design is relatively simple, and it is capable of achieving zero-voltage switching of the switching element, energy recovery of the leakage inductance, suppressing switching surges and other effects; however, its active switching element and the active switching element in the conversion circuit are turned on alternately in each switching cycle, resulting in higher switching loss and poor energy recovery in the active switching element of the active clamp circuit, and its conversion efficiency cannot be improved effectively for converters.
Therefore, there is an urgent need in the art to provide a DC-DC converter that is capable of soft switching, reducing the switching loss of the active switching element in the active clamp circuit, and further improving the efficiency of energy recovery.